Record feeding apparatus and method

ABSTRACT

There is disclosed a method of feeding record webs or sheets and apparatus for carrying out the method. The record feeding apparatus is shown in conjunction with a printing apparatus having a print head assembly, platen structure, a mechanism for severing a printed record from the remainder of the record and an inking mechanism. The record feeding apparatus includes an edge guide, a rotatable feed wheel having a planar frictional surface which engages one face of the web to exert a resultant drive force on the record when driven, the resultant force being comprised of a force vector of large magnitude extending in the longitudinal direction for feeding the record longitudinally and a force vector of small magnitude extending in the lateral direction for causing the record to be driven laterally to cause its side edge to be in guided contact with the edge guide. A roll cooperates with the frictional surface to provide a pinch point disposed either upstream longitudinally of the rotational axis and laterally between the rotational axis and the edge guide or downstream longitudinally of the rotational axis and laterally beyond the rotational axis. A record stepping method and stepping motor and stepping motor control logic are employed to carry out the method to accurately position the record for printing and severing. The stepping motor control logic includes circuitry for advancing the stepping motor either a full increment or one-half of a full increment to provide increased position accuracy without sacrificing drive speed. A current regulated stepping motor driving circuit is used to provide high stepping motor torque without drawing excessive current, and a variable rate stepping pulse generating circuit is employed for gradually accelerating and decelerating the stepping motor. Timout circuitry for disabling the operation of the stepping motor in the event of an abnormal condition is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 537,481, filed Dec. 30, 1974,now U.S. Pat. No. 4,002,277.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to record feeding apparatus and methods.

2. Brief Description of the Prior Art

Prior art Martin, U.S. Pat. No. 2,300,625 and Masterson et al, U.S. Pat.No. 3,350,091 relate to apparatus for feeding sheets longitudinallywhile being guided at a side edge by an edge or side guide. Apparatus ofthis type generally involve a considerable number of parts, gears orpulleys and a special arrangement of bearings.

Various motor control circuits for controlling the feeding of a recordweb through record printing machines are known. Such systems generallyuse a continuously running motor coupled to a web advancing mechanism bymeans of a clutch mechanism which serves periodically to transmit powerfrom the motor to the advancing mechanism for advancing the web. Othersystems may use periodically energized motors or stepping motors foradvancing the web.

Whereas these techniques serve to advance a web through a printingmechanism or the like, systems employing a clutch or periodicallyenergized motors tend to be inaccurate in the positioning of the web inthe printing mechanism. Stepping motor systems require a compromise tobe made between step size and the accuracy of positioning because if thesteps are made small enough to accurately position the web, the rate ofadvance is relatively slow unless excessively high stepping rates areused. If large enough steps to provide an acceptable advancing rate areused, inaccuracies in web positioning result.

SUMMARY OF THE INVENTION

One of the objects of the invention is to overcome the above mentioneddeficiencies and to provide method and apparatus which accomplishes ahigh rate of advance with accompanying improved positioning accuracy.Briefly, a record stepping method and a stepping motor and steppingmotor control mechanism to carry out the method are employed to registerthe record with the print head assembly and the severing mechanism. Morespecifically, the invention provides a novel method for transportingrecords whereby the record is gradually accelerated and decelerated bygradually increasing and decreasing the rate at which the windings of astepping motor are energized, and whereby the record is positioned byenergizing either one or two windings of the stepping motor to providehalf step and full step increments of record travel. Such registrationis accomplished by detecting a notch, or other index disposed on therecord, with a photocell, or the like, and stepping the stepping motor apredetermined number of steps following the detection of the index toaccurately position the record. The stepping motor is driven with aswitching circuit that sequentially energizes the windings of thestepping motor in pairs to incrementally advance the stepping motor. Thesequential energization is controlled by means of a variable speed clockthat has a gradually increasing pulse rate for accelerating the steppingmotor, and a gradually decreasing pulse rate for decelerating thestepping motor. Each of the stepping motor windings is energized bymeans of a switching mode current regulator which provides the drivenecessary to maintain high stepping motor torque without drawingexcessive current from the power supply. In addition, circuitry isprovided for disabling the drive to one of the windings of the energizedpair to provide a half step increment of stepping motor travel toprovide an even more accurate positioning of the record, and timeoutcircuitry is provided to terminate the stepping operation in the eventthat an index is not detected within a predetermined amount of recordtravel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a printing apparatus incorporatingthe record feeding apparatus of the invention,

FIG. 2 is a top plan view of a fragmentary portion of the platenassembly showing a portion of the record feeding apparatus with ahold-down plate;

FIG. 3 is a view similar to FIG. 2 showing a record web in position tobe fed by the record feeding apparatus, but omitting the hold-downplate;

FIG. 4 is an exploded perspective view of the record feeding apparatus;

FIG. 5 is a lateral sectional view of the record feeding apparatus inits assembled condition;

FIG. 6 is an enlarged top plan view similar to FIG. 3, but showing thefeed wheel positioned exaggeratedly relative to the roll for the sake ofclarity;

FIG. 7 is an enlarged top plan view of an alternative embodiment similarto FIG. 6 but showing the feed wheel and the roll positioned differentlyrelative to each other; and

FIG. 8 is a schematic diagram showing the control logic and drivecircuitry for the stepping motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is shown the record feeding apparatusgenerally indicated at 499 adapted for operation with the printingapparatus disclosed in U.S. Pat. No. 3,767,098 to Robert M. Pabodie. Theentire disclosure of U.S. Pat. No. 3,767,098 is incorporated herein byreference. Accordingly, the same reference characters are used in thepresent application as were used in U.S. Pat. No. 3,767,098 to designatelike components.

The drawings of the present application show the record feedingapparatus 499 as feeding a web of records. Consequently, thespecification makes reference to the web, but it is to be understoodthat the invention is also applicable to the feeding of the sheets.Accordingly, the invention is not to be limited to feeding webs but alsoincludes the feeding of sheets. Thus, the expression "record" as used inthe present application includes both webs and sheets.

With reference to FIG. 1 there is shown a frame 31 to which a print headassembly 32 is fixedly mounted. An inking mechanism 35A is used to inkthe print head assembly 32. An electric motor 49 selectively drives adrive shaft 52 through a clutch 51. The drive shaft 52 drives a crank 55which in turn drives a connecting rod 58. A platen assembly 33 pivotallymounted by a pin 34 is driven alternately toward and away from the printhead assembly 32 by the connecting rod 58. A severing mechanism 46 isused to sever record members 37 from the web W. A barrel cam 451 drivesthe inking mechanism 35A through an arm 452. The platen assembly 33 isprovided with a guide plate 108' on which the web W is guided to theprint head assembly 32 and the severing mechanism 46. The platenassembly 33 is provided with a platen frame 108 of which the plate 108'can be considered to be a part. A plate 108" secured to the plate 108'provides an edge guide for the web W.

With reference to FIG. 4 there is shown a stepping motor 500 having adrive shaft 501. A feed wheel generally indicated at 502 is suitablysecured to the drive shaft 501 as by a set screw 503. The feed wheel 502is rotatable in an enlarged hole 502' in the plate 108'. The motor 500is secured to a motor mount 504. The motor 500 has a flange 505. Screws506 (only one of which is shown in FIG. 4) pass through four holes 507(only three of which are shown) and are received in threaded holes 508in the motor mount 504. A bracket 509 is secured to the plate 108' byscrews 510 (only one of which is shown). The screws 510 extend throughholes 511 in the plate 108' and extend through cutouts 512 in the bightportion 513 of the bracket 509. A nut plate 514 has threaded holes 515which receive the respective screws 510. The nut plate 514 clamps thebight portion 513 against the underside of the plate 108'. The bracket509 includes depending arms 516 joined to the bight portion 513. Thearms 516 have respective holes 517 for receiving a pivot pin 518. Thepivot pin 518 passes through holes 519 in the motor mount 504 and isreceived in the holes 517. Accordingly, the pivot pin 518 and thebracket 509 mount the motor mount 504 and hence the motor 500 and thefeed wheel 502 which it carries for pivotal movement. The pivot pin 518is held in position by a retainer 520, the curved end 521 of which isreceived in an annular groove 522 on the outer surface of the pivot pin518. A screw 522' is threadably received by the arm 516 at the left sideof the bracket 509 as viewed in FIG. 4.

A counterweight 523 is secured to the underside of the motor mount 504by a screw 524 which passes through a hole 525 and is threadablyreceived in a threaded hole 526 in the motor mount 504. As best seen inFIG. 1, most of the mass to be counterbalanced resides in portion thestepping motor 500 which is disposed to the right of the pivot pin 518.The counterweight 523 is relatively long and essentially counterbalancesall of the mass disposed to the right of the pivot pin 518 as viewed inFIG. 1. FIG. 1 shows the platen assembly 33 in its upward position sothat one of the record members 37, defined by lines of partial severing37', is in printing cooperation with the print head assembly 32. It willbe appreciated that as the platen assembly 33 pivots downwardly awayfrom that position, the amount of the mass required to becounterbalanced decreases which means that the feed wheel 502 will exerta greater upward force against the underside of the web W. However, thisincrease in force is slight and is well within the range of maximumdesired for this purpose. A compression spring 526' received on a post527 secured to the counterweight 523 exerts a downward force on thecounterweight and an opposite upward force on the underside of the plate108'. The purpose of the spring 526 is to insure that there is alwaysadequate force being applied by the feed wheel 502 to the underside ofthe web W and to compensate for any bounce that may result when theplaten assembly 33 starts and stops during the printing cycle.

A roll 528 is a needle bearing mounted on the shaft 530. The onemarginal end of the shaft 530 is received in a bushing 531 received in abracket 532. The bracket 532 is suitably secured to the platen frame108. A set screw 533 is threaded into the bracket 532 and extendsthrough a hole 534 in the bushing 531 and bears against the shaft 530.The other end of the shaft 530 is received by a rolled end 535 of a webhold-down plate 536. The plate 536 is locked to the one arm 516 by aretainer 537 and a set screw 538. The retainer 537 and a washer 539 arereceived by a stud 540 having a threaded end 541 received in onedepending arm 516. The feed wheel 502 is shown to have a frictionalsurface generally indicated at 542. The frictional surface 542 is morespecifically shown to comprise an annular ring having a multiplicity ofsmall serrations 543 which extend in a radial direction as best shown inFIG. 6. The portion of the wheel 502 within the annular ring is recessedas indicated at 542'. Axis 501' of the shaft 501 is also the rotationalaxis of the feed wheel 502. It is apparent that the frictional surface542 rotates in a plane that is perpendicular to the axis 501'. It isalso apparent from FIG. 6 that the plane in which the frictional surface542 rotates is parallel to face W' of the web W. The web W is disposedbetween the feed wheel 502 and the roll 528. More particularly the roll528 exerts pressure against the other face W" of the web W in oppositionto the force exerted by the frictional surface 542 at the pinch point P.

The web W is fed in the direction of arrow A as shown in FIGS. 3 and 6in the downstream direction. As best shown in FIG. 6 the pinch point Pis disposed upstream of the rotational axis 501' by a distance D. Inaddition the pinch point P is disposed laterally between the rotationalaxis 501' and edge E of the guide 108". The resultant force indicated byvector VR is comprised of a force vector VR1 of large magnitudeextending in the longitudinal direction for feeding the weblongitudinally and a force vector VR2 of small magnitude extending inthe lateral direction for driving the web W laterally to cause its sideedge se1 to be in contact with edge E with concomitant slippage betweenthe frictional surface 542 and the side W' of the web W to prevent theweb from buckling laterally. The force vector VR extends tangentially tothe frictional surface 542 of the wheel 502. Although shown exaggeratedin FIG. 6, the force vector VR2 is very small compared to the forcevector VR1. Nevertheless, if there were no slippage between thefrictional surface 542 and the web W, the web would buckle somelaterally between the pinch point P and the side edge se1 of the web W.In that the vector VR2 is relatively small and the web W which iscomprised of tag stock is relatively stiff and because of the smallamount of slippage that takes place, the side edge se1 of the web W isalways in contact with and guided by the edge E of the guide 108" as theweb W is driven in the direction of arrow A.

In order to facilitate threading of the web W through the printingapparatus and specifically to facilitate insertion in the web W betweenfeed wheel 502 and the roll 528 during the threading process, there isprovided a lever 544 which is pivotally mounted on a pivot pin 545. Thepivot pin 545 is secured to the frame 108 by a threaded portion 546. Thelever 544 is held on the pivot pin 545 by a retainer 546. The lever 544has a manually engageable surface or button 547. As viewed in FIG. 5,depressing the button 547 pivots the lever 544 counterclockwise in thatthe lever contacts the motor mount 504, the motor mount 504 and themotor 500 and the feed wheel 502 which it pivots clockwise (FIG. 1)about the pivot pin 518. With the button 547 held depressed the feedwheel 502 is away from the roll 528 sufficiently to allow the free endof the web W to be easily inserted between the feed wheel 502 and theroll 528. Once this has been accomplished the button 547 can be releasedand the counterweight 523 and the spring 526 will urge the surface 542of the wheel 502 into pressure contact with face W'. In that thepinching force exerted on the web W at the pinch point P is highrelative to the force exerted by the frictional surface 542 on the faceW' at other than the pinch point P, almost all the driving forcetransmitted to the web W takes place at the pinch point P. The forcestransmitted to the web W by the frictional surface 542 at other than thepinch point P do not interfere with reliable feeding of the web W. Thepinch point P is not a finite point but rather it is the area of the webwhich experiences the pressure exerted by the action of the frictionalsurface 542 and the roll 528.

In the embodiment of FIG. 7, the web W is fed in the downstreamdirection indicated by arrow A1. The pinch point P1 is disposeddownstream of the rotational axis 501' by a distance D1. In addition thepinch point P1 is disposed laterally beyond the rotational axis 501'.The resultant force indicated by vector VR3 is comprised of a forcevector VR4 of large magnitude extending in the longitudinal directionfor feeding the web longitudinally and a force vector VR5 of smallmagnitude extending in the lateral direction for driving the web Wlaterally to cause its side edge se1 to be in contact with edge E withconcomitant slippage between the frictional surface 542 and the side W'of the web W to prevent the web from buckling laterally. The forcevector VR3 extends tangentially to the frictional surface 542 of thewheel 502. Although shown exaggerated in FIG. 7, the force vector VR5 isvery small compared to the force vector VR4. Nevertheless, if there wereno slippage between the frictional surface 542 and the web W, the webwould buckle some laterally between the pinch point P1 and the side edgese1 of the web W. In that the vector VR5 is relatively small and the webW which is comprised of tag stock is relatively stiff and because of thesmall amount of slippage that takes place, the side edge se1 of the webW is always in contact with and guided by the edge E of the guide 108"as the web W is driven in the direction of arrow A1. In the embodimentof FIG. 7 the motor rotates in the opposite direction from the directionof rotation of the motor 500 in the embodiment of FIGS. 1 through 6.

If it is desired to change the position of the pinch point P or P1 whichwill thus change the respective vectors VR1 and VR2, or VR3 and VR4 thiscan be accomplished by loosening the screws 510 and shifting the bracket509 longitudinally. The cutouts 512 are large enough so that the bracket509 can also be shifted laterally. Upon movement of the bracket 509 tothe selected position and tightening of the screws 510 the bracket 509is secured in the selected position and thus the feed wheel 502 will bein the desired position relative to the roll 528.

Stepping motor 500 and stepping motor control circuit are utilized toaccurately position the record to be printed under the print headassembly. In this embodiment, the stepping motor 500 has 200 steps perrevolution, or 1.8° per step, is employed to advance the record throughthe printing apparatus. Feed wheel 502, mechanically coupled to thestepping motor, is utilized to drive the record, and the diameter of thefeed wheel is selected so that one step of the motor results inapproximately 13.3 mils of record travel.

In general, the control logic and drive circuitry for the stepping motor500 comprises a photocell circuit 610, a feed circuit 612, a stepcounting circuit 614, a step rate control circuit 616 comprising anoscillator 618 and a slew control circuit 620. A motor driving circuitincluding a sequencing circuit 622 is used for alternately energizingfour stepping motor windings 624, 626, 628 and 630.

The record web W is fed into the printing apparatus as a continuousstrip having a plurality of notches 37n or other indices formed in oneedge thereof to define the spacing between the record members 37. Thesenotches 37n are sensed by a photocell (not shown) so that the positionof each record member 37 with respect to the print head assembly may bedetermined and the feeding operation terminated when a record member 37is in the printing position.

The output signal from the index detecting photocell is applied to aninput point 632 of the photocell circuit 610 where it is squared by aSchmitt trigger comprising three NAND gates 634, 636 and 638 prior tobeing applied to the rest of the logic circuitry. The squared pulsesfrom the photocell circuit 610 are applied to a seven stage binarycounter 640 within the counting circuit 614 together with oscillatorpulses from the oscillator circuit 618. The pulse from the photocellcircuit 610 serves to reset the counter 640, and the number ofoscillator pulses returned from the OSC output of the oscillator 616subsequent to the receipt of each pulse from the photocell circuit 610are counted by the counter 640 to determine the position of each ticket.The COUNT 16 output is applied to the feed circuit 612 to terminate thefeeding of the web W after the record member 37 has been advanced intothe printing position. This occurs after the stepping motor has beenadvanced approximately 16 steps following the detection of a notch 37n.A 3-input NAND gate 665 is connected to the COUNT 8, COUNT 32 and COUNT64 outputs of the counter 640 to provide a COUNT 104 timeout signal toterminate the operation of the printing apparatus if no photocell signalhas been received after 104 counts (8 + 32 + 64).

The speed at which the stepping motor 500 is stepped is determined bythe frequency of the oscillator 618. Because of the inertia of thestepping motor 500 and the feed wheel 502, it is desirable initially tostep the motor at a relatively slow, gradually increasing, rate until amaximum rate is achieved, and then to gradually slow the rate prior tostopping the web W for printing and severing. This is accomplished bycontrolling the oscillator 618, which is a variable rate oscillatorhaving a maximum pulse rate of approximately 700 pulses per second, by aslew generator 620 that initially increases the pulse rate of theoscillator 618 at a gradual rate, and then gradually decreases the pulserate before the oscillator 618 is turned off.

The pulses from the oscillator 618 control the switching circuit 622 foralternately energizing the windings 624, 626, 628 and 630 of thestepping motor to provide the incremental stepping action. A half stepgenerator (described in a subsequent portion of the specification)within the switching circuit 622 compensates for errors resulting fromthe lack of synchronization between the oscillator 618 and the output ofthe photocell circuit 610, and permits the stepping motor 500 to beadvanced either 15.5 or 16 steps following the receipt of a photocellpulse, depending on the relative phase of the oscillator signal and thephotocell pulses. In addition, a pair of current limiting switchingtransistors 644 and 646 limit the maximum amount of drive current thatcan be applied to the windings 624, 626, 628 and 630 while providing ahigh initial drive for rapidly accelerating the stepping motor.

Referring to FIG. 8 in greater detail, a master reset pulse MR isapplied to an input point 643 when power is applied to the apparatus.The MR pulse resets all of the logic and prevents the stepping of theweb W until an initiate pulse is applied to an input point 658 of thefeed circuit 612. The initiate pulse is generated by a cam lobe (notshown) on the printer drive shaft 652. The initiate pulse applied to theinput 658 toggles a latch formed by a pair of gates 645 and 647 toinitiate the feeding operation. The photocell scans the advancing web W,and whenever an index notch 37n is detected, a pulse is applied througha diode 648 to the NAND gate 634 which is connected as an invertingamplifier. The NAND gates 634, 636 and 638 serve as a Schmitt trigger tosquare up the pulse from the photocell, and the diode 648, inconjunction with a potentiometer 650 and a capacitor 652 serve as avariable delay network for the photocell pulse. The variable delaynetwork permits a vernier adjustment of the web position to be made byadjusting the potentiometer 650.

The output from the NAND gate 636 is coupled to a NAND gate 654 by meansof a resistor 655 and a coupling capacitor 656. The NAND gate 654 has asecond input connected to the output 658 and provides an output pulsefor resetting the counter 640 upon receipt of a low state initiate pulseCTB at the input point 658 and a low state photocell pulse (indicatingthe presence of a notch 37n) from the NAND gate 636. A resistor 657 anda diode 659 return the input of the gate 654 to a positive potential toterminate the reset signal if the output of the gate 636 remains low anexcessively long time. An excessively long low state condition can occurif no web W being fed through the apparatus (e.g. after all recordsmembers 37 in a web W have been printed), and it is necessary toterminate the reset signal to allow the apparatus to turn offautomatically under such a condition.

The OSC output from the oscillator 618 is coupled to the clock input ofthe counter 640 which operates both as a timeout counter and as a recordpositioning counter. The counter 640 counts the number of oscillatorpulses received subsequent to the receipt of the trailing edge of theCTB pulse from the NAND gate 654.

Initially, the counter 640 operates in a timeout mode to terminate thefeeding of the record if 108 pulses are received subsequent to thereceipt of the CTB pulse from the NAND gate 654. If a photocell pulse isreceived from the NAND gate 636 (via NAND gate 654) prior to the receiptof 108 pulses from the oscillator 618, the counter 640 is again reset,and the feeding of the record is terminated 16 pulses after the receiptof the trailing edge of the photocell pulse.

The timeout feature is provided by a NAND gate 665. The COUNT 8, COUNT32 and COUNT 64 outputs are applied to the NAND gate 625 which providesa low output upon the coincidence of the COUNT 8, COUNT 32 and COUNT 64outputs. The output from the NAND gate 665 is inverted by a NAND gate666 which is connected as an inverting amplifier to provide a COUNT 104signal upon receipt of the low output from the NAND gate 665. The COUNT104 signal is applied to the set input S of a photocell flip-flop 668and to the reset input R of a timeout flip-flop 670. The timeoutflip-flop 670 provides a TMO signal at an output 672 that goes low toindicate that a timeout has occurred. The Q output of the photocellflip-flop 668 is applied to a NAND gate 674 to terminate the productionof pulses by the oscillator circuit 616. This prevents further steppingof the stepping motor and advancement of the record following thetimeout.

If a photocell pulse is received within the 108 counts, the counter 640is again reset by the trailing edge of the photocell pulse received fromthe NAND gate 636. The counter 640 again begins to count from zero, andthe COUNT 16 output of the counter 640 is applied to a NAND gate 660 forresetting a feed flip-flop 662 via a second NAND gate 664. The flip-flop662 generates FEED and FEED signals for terminating the feeding ofrecords after 16 pulses have been received from the oscillator 618. TheFEED signal is applied to the oscillator 618 to terminate the generationof pulses, and the FEED signal is applied to the NAND gate 674 to reducethe oscillator pulse rate for decelerating the stepping motor prior toturn off.

The sequence of energization of the stepping motor coils 624, 626, 628and 630 is controlled by a pair of flip-flops 676 and 678 which areclocked by pulses from the oscillator 616. The Q output of the flip-flop676 is connected to the data input D of the flip-flop 678 and the Qoutput of the flip-flop 678 is connected to the data input D of the flipflop 676. Consequently, the Q output of the flip-flop 676 is clockedinto the flip-flop 678, and the Q output of the flip-flop 678 is clockedinto the flip-flop 676 in synchronism with the clock pulses from theoscillator 616. The Q and Q outputs from the flip-flops 676 and 678 areinverted by four inverting amplifiers 680, 682, 684 and 686 whichprovide A, A, B and B outputs to four switching transistors 688, 690,692 and 694 via four coupling resistors 696, 698, 700, and 702,respectively. The four inverting amplifiers 680, 682, 684 and 686provide an output sequence of AB, AB, and AB for sequentially renderingthe transistors 688, 690, 692 and 694 conductive as pairs 688, 692; 690,692; 690, 694; and 688, 694 and for sequentially energizing the pairs ofwindings 624, 628; 626, 628; 626, 630; and 624, 630. The sequentialenergization of the winding pairs causes the stepping motor to bestepped one full step each time a successive pair of windings isenergized.

Because the windings 624, 626, 628 and 630 are highly inductive, thecurrent through the windings builds up at a gradual rate determined bythe voltage of the power supply and the inductance of the windings.Therefore, in order to provide adequate current to assure sufficientstepping motor torque, it is desirable to apply the entire 28 voltoutput of the printer power supply directly across the windings 624,626, 628 and 630. However, because the windings 624, 626, 628 and 630present an inductive load to the power supply, the windings can drawexcessive current after they have been connected across the power supplyfor a sufficient period of time for the switching transient to decay.Accordingly, the current limiting transistors 644 and 646 are employedto limit the maximum value of current that can flow through the windings624, 626, 628 and 630. The current limiting transistor 646 works inconjunction with the switching transistors 688 and 690 to apply currentto the windings 624 and 626, and the current limiting transistor 644works in conjunction with the transistors 692 and 694 to apply currentto the windings 628 and 630. Because not more than two windings are everenergized at any one time, each of the current limiting transistors 644and 646 is shared with each current limiting transistor alternatelysupplying current to one of the two simultaneously energized windings.

Both of the current limiting transistors 644 and 646 are normallysaturated, and the current flowing through each of the transistors 644and 646 is channelled to one of the windings by one of the switchingtransistors 688, 690, 692 and 694. The current flowing through thewindings 624 and 626 is sensed by a current sensing resistor 704connected in series with the emitters of the switching transistors 688and 690, and the current flowing through the coils 628 and 630 is sensedby the current sensing resistor 106 connected in series with theemitters of the switching transistors 692 and 694. The current sensingresistors 604 and 606 are connected across the inputs of a pair ofrespective operational amplifiers connected as Schmitt triggercomparators 708 and 710 by means of coupling resistors 712, 714, 716 and718. The Schmitt trigger comparators 708 and 710 sense the voltagesacross the respective resistors 704 and 706 and provide an output signalto a respective one of the current limiting transistors 646 and 644 torender the current limiting transistor nonconductive when the voltageacross the respective one of the resistors 704 and 706 exceeds apredetermined level. Two resistors 709 and 711 are used to provide thecomparators 708 and 710 with a predetermined amount of hysteresis, and apair of speed up capacitors 713 and 715 are used to speed up theswitching action.

Because of the inductive nature of the stepping motor windings 624, 626,628 and 630, a high voltage inductive surge would be generated each timeone of the transistors 644 and 646 was switched to its nonductive stateif an alternate current path were not provided. A pair of diodes 720 and722 are utilized to provide this alternate path. The diode 720 isconnected across each of the series circuits comprising the windings624, 626 and the transistors 688, 690 to permit the current to continueflowing through the one of the windings 624, 626 that had been energizedprior to the turning off of the current limiting transistor 646. Thediode 722 provides a similar function for the windings 628 and 630. As aresult, a gradual decaying current continues to flow through thewindings after the current limiting transistors 644 and 646 have beenswitched off. The current through the windings continues to decay untilthe voltage across one of the current sensing resistors 704 and 706drops below a predetermined level which causes the respective one of thecomparators 708 and 710 to change state again and to render therespective one of the switching transistors 646 and 644 conductive. Thisswitching of the transistors 644 and 646 occurs at a rate ofapproximately 7000 Hz.

Because of the relatively high currents being switched, the 7000 Hzswitching signal can cause undesirable interference and transientsuppressing circuitry including resistors 714 and 718 and capacitors720, 722 and 724 is used to minimize the interference thus generated. Inaddition, diodes 726, 728, 730 and 732 are connected to the respectiveswitching transistors 688, 690, 692 and 694 to an anode of a Zener diode734 to limit the voltage transient at the collectors of the switchingtransistors to a maximum of approximately 50 volts. Four diodes 736,637, 740 and 742 are used to prevent any reverse voltage transient frombeing applied to the collectors of the switching transistors, and toprevent reverse current from flowing through the transistors 688, 690,692 and 694.

Because the oscillator 616 is not synchronous with the pulses providedby the photocell circuit 610, the amount of linear web travel subsequentto the receipt of a photocell pulse is dependent upon the relative timeof receipt of the signal from the photocell circuit 610 and the outputpulses from the oscillator 616. For example, if the photocell signal isreceived just prior to an oscillator pulse, the distance subsequentlytraveled by the web corresponds to 16 steps of the stepping motor. Ifthe photocell signal is received just after an oscillator pulse, nearly17 steps of web travel results. Thus, a nonuniformity between recordmembers 37 corresponds to one stepping motor increment plus other errorscaused by feeding apparatus 499 tolerances can result.

In order to minimize such errors, a circuit for generating half stepincrements is provided. The half step increments are generated bysuppressing the energization of one of the windings to generate a halfstep sequence AB, B, AB, A, AB, B, AB, A. The half step increment isused only for the last step of the sequence and is enabled by three NANDgates 736, 738, and 740 which function as an exclusive OR for disablingthe drive to one of the windings if A and B are both 1 or both 0. Theoutput of the NAND gate 740 is applied to a pair of three input NANDgates 742 (via NAND gate 743) and 744. The NAND gates 742 and 744 areconnected to the respective comparators 708 and 710 for disabling therespective one of the current switching transistors 646 and 644 uponreceipt of a signal from the NAND gate 740 indicating that A and B areboth 1 or both 0, the FEED signal from the feed flip-flop 662 indicatingthe sixteenth step, and the Q output of a half step flip-flop 746indicating that the photocell signal was received shortly before theoscillator pulse.

The half step flip flop 746 measures the time of arrival of thephotocell signal in relation to the stepping pulse from the oscillatorcircuit 616. If the photocell signal is received before the midpoint ofthe oscillator signal, an output for disabling one of the currentlimiting transistors 46 and 44 is provided, and the motor is advancedonly one half step to provide a total of only 15.5 steps of advance. Ifthe photocell signal is received during the last half of the oscillatorpulse, the stepping motor 500 is advanced a full 16 steps. As discussedpreviously, the NAND gates 742 and 744 permit the current limitingtransistors 746 and 744 to be disabled only during the last step of thesixteen step sequence, as indicated by the FEED output of the feedflip-flop 662.

The oscillator circuit 618 comprises a relaxation oscillator including acomparator 750 and associated biasing circuitry and a capacitor 752. Thecapacitor 752 is charged from a 12 volt source by means of resistors754, 756, and 758. Biasing resistors 760, 762 and 764 provide areference voltage to the (+) input of the comparator 750 to which thevoltage across the capacitor 752 is compared. As the capacitor 752charges through the resistors 754, 756 and 758, the capacitor voltageincreases until a level greater than the level of the voltage applied tothe (+) terminal of the comparator 750 is reached. At this point, theoutput of the comparator 750 changes state (goes low), and the capacitor752 is discharged through the resistor 758 and a diode 766. After thecapacitor 752 has been discharged to a level below that provided by thebiasing resistors 760, 762 and 764, the operational amplifier 750changes state again and the charging cycle is repeated. The resistor 762determines the hysterisis of the comparator 750 and hence the level towhich the capacitor 752 must discharge before the comparator againchanges state. A capacitor 753 serves as a noise filter at the input ofthe comparator 750.

The frequency of oscillation is controlled by the ramp generator 620.The ramp generator 620 comprises a comparator 770 which provides agradually increasing voltage at its output upon receipt of a FEED and aPC signal at the inputs of the NAND gate 674. The output signal from thecomparator 770 is applied to the junction of the resistors 754 and 756(and a stabilizing capacitor 771) through a resistor 773. The signalfrom the amplifier 770 changes the voltage at the junction of theresistors 754 and 756 and varies the rate at which the capacitor 752 ischarged, and hence, the rate of oscillation of the oscillator 618.Initially, the output voltage of the amplifier 770 is low, therebyproviding a slow charging rate for the capacitor 752 and a relativelylow oscillation rate for the oscillator 618. As the voltage at theoutput of the comparator 770 increases (as a result of the dischargingof the capacitor 772 through a resistor 774), the oscillation rate ofthe oscillator 618 increases until its maximum rate of approximately 700Hz. is reached. This oscillation rate is maintained, until the FEED (orPC) signal is removed. At this point, the output voltage of theamplifier 770 gradually decreases as the capacitor 772 is charged to alevel determined by a biasing network including resistors 776 and 778and the diodes 780 and 782. Removal of the FEED signal also causes aFEED signal to be applied to the (+) input of the comparator 750 througha resistor to raise it to a level that cannot be exceeded by the voltageacross the capacitor 752, thereby preventing further oscillation.

Other embodiments and modifications of this invention will suggestthemselves to those skilled in the art, and all such of these as comewithin the spirit of this invention are included within its scope asbest defined by the appended claims.

I claim:
 1. A control system for a stepping motor having a drive shaftand a plurality of windings, the sequential electrical energization ofsaid windings being effective to rotate said drive shaft inpredetermined increments, the control system comprising: means foralternately energizing the windings in a predetermined sequence, theenergizing means including means for altering the sequence to change themagnitude of the increments; and means connected to said energizingmeans for determining the rate at which the windings are energized, saidrate determining means including means for gradually increasing theenergizing rate for accelerating the stepping motor and for graduallydecreasing the energizing rate for decelerating the stepping motor.
 2. Acontrol system as defined in claim 1, wherein the energizing meansincludes means for limiting the current through the windings.
 3. Acontrol system as defined in claim 2, wherein the current limiting meansincludes a switching current regulator.
 4. A control system as definedin claim 3, wherein the stepping motor includes four windings and theenergizing means includes four switch means, one of the switching meansbeing connected in series with each of the windings.
 5. A control systemas defined in claim 4, wherein the current limiting means includes asecond switching current regulator, the switching current regulatorbeing connected in circuit with two of the switch means for alternatelysupplying current to two of the windings, and the second switchingcurrent regulator being connected in circuit with the other two of theswitch means for alternately supplying current to the other two of thewindings.
 6. A control system as defined in claim 1, wherein theenergizing means includes logic means for rendering the energizing meansoperative for sequentially energizing the windings in pairs.
 7. Acontrol system as defined in claim 6, wherein the logic means includesmeans for suppressing the energization of one winding of one of theenergized pairs in the predetermined sequence.
 8. A control system for astepping motor having a plurality of windings, comprising: means foralternately energizing the windings in groups of at least two in apredetermined sequence; and means responsive to an external conditionfor suppressing the energization of one winding of one of the groups inresponse to the external condition.
 9. A control system as defined inclaim 8, further including pulse producing means for rendering theenergizing means operative to energize the successive groups of windingsof the predetermined sequence in response to a pulse therefrom, andwherein the external condition responsive means is responsive to therelative time of occurrence of the external condition and the pulse fromthe pulse producing means.
 10. A control system as recited in claim 8,wherein the stepping motor includes a drive shaft and is responsive tothe successive energization of the windings in groups for incrementallyrotating the drive shaft in first predetermined increments and isresponsive to the suppression of the energization of the one winding forrotating the drive shaft a second predetermined increment less than saidfirst predetermined increments.
 11. A control system as recited in claim8 wherein said external condition is the position of a workpiece.